GB2544793A

GB2544793A - Cryogen pumping system

Info

Publication number: GB2544793A
Application number: GB1520991.9A
Authority: GB
Inventors: Matthews Anthony
Original assignee: Oxford Instruments Analytical Ltd
Current assignee: Oxford Instruments Analytical Ltd
Priority date: 2015-11-27
Filing date: 2015-11-27
Publication date: 2017-05-31
Anticipated expiration: 2035-11-27
Also published as: GB2544793B; GB2544793A8; GB201520991D0

Abstract

Disclosed is a cryogen pumping system 1 comprising a cryogen pump 10 with an inflow element 12, an outflow element 14 and a conduit 30 connected between the inflow element and the outflow element and comprising a flow restriction 34. The flow restriction is configured to limit the rate of cryogenic fluid flow through the conduit to less than the rate of flow producible by the pump when in use so as to maintain an operational pressure differential in the cryogenic fluid between the inflow element and outflow element. The flow restrictor may be a throttle, a static flow impedance, a narrower bore diameter or a sheet with an orifice. Preferably the flow restriction allows fluid to flow from the outflow side of the pump to the inflow side of the pump when the pump is inactive so as to equalise any pressure differential across the pump that may cause the pump to stall on start-up. Also claimed is a dilution refrigeration system and a cryostat system.

Description

Cryogen Pumping System

Field of the invention

The present invention relates to a cryogen pumping system, in particular to pressure equalisation across a cryogen pump in a cryogen pumping system, such as those used in cryogenic cooling systems.

Background

In systems using cryogens, it is common for a cryogen to be cycled through the system to provide cooling. This is achieved by passing the cryogen through ducts, such as pipes, stills and other chambers. To circulate the cryogen, a cryogen pump is used.

When passing through the pump, the cryogen is in the gas phase, but in other parts of the system, the cryogen may be cooled by an external cold source. As a result of this, the cryogen is a liquid in some of the sections of the system through which it passes.

To ensure reliable operation of the system, the cryogen pump needs to be an oil-free pump so that the cryogen and the components that the cryogen comes into contact with are not contaminated. This limits the type of pump that can be used in such a system.

The pipes through which the cryogen is passed have a high flow impedance, as they have a relatively small diameter. For example, some of the pipes have a diameter of about 1 millimetre (mm). Additionally, the cryogen is also passed through a number of flow impedances and heat exchangers. This places a further limitation on the system, which is that the cryogen needs to be at high pressure for it to be passed through the system.

The need to pass the cryogen through the system at high pressure also means that the pump has to compress the cryogen passing through it. Due to the need to keep cryogens contained, the pump also needs to prevent cryogen gas leaks. Suitable pumps that fulfil all these criteria are some diaphragm pumps, as well as any pump that prevents cryogen gas leaks to the surrounding environment while also being able to achieve outflow pressures up to about 4 bar.

However, the combination of a gas tight seal and the higher cryogen pressure on the outflow side of the cryogen pump than on the inflow side of the cryogen pump, can cause the pump to stall on activation (i.e. when the pump is started). This leads to the pump overheating and, eventually, to burn out of the pump’s motor.

There is therefore a need to provide a means of avoiding the pump stalling on activation.

Summary of Invention

According to a first aspect, there is provided a cryogen pumping system, comprising: a cryogen pump with an inflow element and an outflow element; and a conduit comprising a flow restriction, the conduit being connected between the inflow element and the outflow element, wherein the flow restriction is configured to limit the rate of cryogenic fluid flow through the conduit to less than the rate of cryogenic fluid flow producible by the cryogen pump when in use so as to maintain an operational pressure differential in the cryogenic fluid between the inflow element and outflow element produced by the cryogen pump when in use.

This allows any pressure differential across the pump to equalise when the pump is inactive but maintains the pressure differential and high pressure needed on the outflow side of the pump when the pump is active (i.e. when the pump is in use) and pumping cryogen through the system. This is achieved because the flow rate of cryogen from the outflow side of the pump through the conduit to the inflow side of the pump either does not reach this limiting condition, and/or has sufficient time to equalise when the pump is inactive (i.e. when the pump is not in use). When the pump is active, the limiting of the cryogen flow rate through the conduit allows the pressure to increase on the outflow side of the pump so that the impedance of the system pipes through which the cryogen is to be passed is able to be overcome.

Accordingly, this removes the pressure differential, which might otherwise cause the pump to stall on activation, whilst still allowing high pressures to be achieved when the pump is in use to allow the cryogen to be passed through the system.

The cryogen pump may prohibit internal fluid flow between the inflow element and outflow element when inactive. This optional feature avoids the possibility of cryogen passing through the pump in the wrong direction.

The flow restrictor may be any mechanism that allows the flow rate through the conduit to be restricted. For example, the flow restrictor may be a throttle. This provides a convenient means of regulating the cryogen flow rate through the flow restrictor.

The flow restrictor may be an active component such as a needle valve. However, typically, the flow restrictor is a static flow impedance. A static impedance allows the flow restrictor to be a passive component that will function without the need to be controlled, which means the flow restrictor functions at all times and it requires less maintenance than active components.

The flow restrictor may be a section of the length of the conduit with a narrower bore diameter than the other sections of the length of the conduit. This provides the ability to choke the flow, which restricts the mass flow rate through the conduit (i.e. the rate of mass of cryogen that passes through the conduit per unit time). This is a self-establishing effect as the flow rate increases, which allows complete passive flow impedance through the conduit, so no external intervention is required.

Alternatively, the flow restrictor may be a sheet positioned across a bore of the conduit with an orifice therethrough, the orifice having a smaller cross-sectional area than the cross-sectional area of the bore of the conduit. This provides a restriction on the rate of flow through the conduit while avoiding a fully choked flow at any flow rate, which can be advantageous in some circumstances, and is simple to manufacture.

The conduit may be directly connected to each of the inflow element and the outflow element, or indirectly, such as by connection to a first chamber connected to the inflow element and a second chamber that is connected to the outflow element. Typically though, the conduit is connected to the inflow element by an inflow pipe configured to supply cryogen to the pump when in use and is connected to the outflow element by an outflow pipe configured to transport cryogen away from the pump when in use. This provides an indirect connection between the conduit and each of the inflow and outflow elements while providing a direct connection to the pipes that supply the cryogen to the pump and into which the pump passes cryogen. It is between these pipes where the pressure differential is most extreme, so providing a direct connection between these pipes has the greatest effect on the pressure differential across the pump. Additionally, this allows the conduit to be retro-fitted, as these pipes are the most accessible part of the cryogen pump system, making retro-fitting most simple.

According to a second aspect, there is provided a dilution refrigeration system, comprising: a dilution refrigerator; and a cryogen pumping system according to the first aspect comprising an inflow element and an outflow element, wherein the inflow element is connected to a cryogen outflow of the dilution refrigerator and the outflow element is connected to a cryogen inflow of the dilution refrigerator.

According to a third aspect, there is provided a cryostat system, comprising: a cryostat; and a dilution refrigeration system according to the second aspect located within the cryostat and configured to provide cooling to at least a part of the cryostat.

Brief description of the drawings

Examples of a cryogen pumping system are described in detail below, with reference to the accompanying drawings, in which:

Figure 1 shows a partially exploded perspective view of a first embodiment of the cryogen pumping system;

Figure 2 shows a graph comparison of the flow rate vs pressure of an example cryogen pumping system without a conduit to equalise pressure with the flow rate vs pressure of a cryogen pumping system according to the first embodiment;

Figure 3 shows a partial sectional view of a schematic of the first embodiment of the cryogen pumping system;

Figure 4 shows a graph of the ratio of pressure in cryogen conduits against the ratio of cryogen flow velocity to sonic velocity of the cryogen through an example flow restrictor; and,

Figure 5 shows a partial sectional view of a schematic of a second embodiment of the cryogen pumping system.

Detailed description

According to the embodiments described here, in each case, a cryogen pumping system is connected to a dilution refrigerator. Dilution refrigerators are designed to provide cooling at temperatures as low as about 2 milliKelvin (mK), known as the operational base temperature. The efficiency of dilution refrigerators increases as the temperature of the dilution refrigerator approaches the helium-3/helium-4 phase separation point at about 0.7 Kelvin (K), and then decreases as the temperature of the dilution refrigerator is lowered further towards the operational base temperature. Accordingly, dilution refrigerators need cooling before they can be used to apply cool themselves.

The principle of operation of a dilution refrigerator is well known, and can broadly be separated into four modes. These are a pre-cooling mode, condensing mode, normal operation mode, and high cooling power operation mode.

In the pre-cooling mode, the dilution refrigerator is cooled from room temperature to close to its operational base temperature. To achieve this, a cryogen, which in this case is a mixture of helium-3 (3He) and helium-4 (4He), is pumped around the dilution refrigerator, or through a separate pre-cooling circuit, at high flow rates (for example at about 75 litres per minutes (L/min) at Standard Temperature and Pressure (STP)).

To cool the cryogen, the cryogen is first passed along a path from a cryogen pump (or storage dump), through an outlet pipe to a cooling stage, such as pulse tube cooler, a stage cooled by liquid cryogens, or some other form of cryocooler. The cryogen then cools the dilution refrigerator by passing through the rest of the dilution refrigerator. In essence, this involves then passing the cryogen into a condenser, through a primary impedance, and then through a secondary impedance and heat exchangers into a mixing chamber. Once the cryogen reaches the mixing chamber, it passes through a duct into a still, through an outflow of the dilution refrigerator into a number of further pumps and into an inlet pipe for the cryogen pump and (back) into the pump to be cycled through the dilution refrigerator again.

The flow impedances are either sintered stainless steel powder, a capillary system, a needle valve or some other form of flow impedance, and the heat exchangers cause the cryogen to pass through metal foam. Additionally, the pipe diameters are predominantly of the order of about 1 mm. As such, high pressures, such as about 3 bar at the outflow of the cryogen pump, are needed to drive the cryogen through the dilution refrigerator.

In the condensing mode, cryogen along much of the path detailed above is condensed into a liquid. As such, the condensation process results in a gas-liquid interface at the condenser and the still so that there is only gas in the still, outflow and inflow pipes of the dilution refrigerator, and the cryogen pump system. As an example of the pipe sizes of the dilution refrigerator, the outflow pipe has a diameter or about 100 mm, and the inflow pipe has a diameter of about 1 mm.

Together with the impedance imposed on the flow of cryogen by the dilution refrigerator, the small diameter of the inflow pipe results in a high resistance (i.e. high impedance) to the flow of cryogen, which means that high pressure, such as about 3 bar at the outflow of the cryogen pump, is needed to push the cryogen through the outflow pipe.

Once the cryogen is condensed, the dilution refrigerator is able to be used to cool other items in either the normal operation mode or the high cooling operation power mode. During normal operation, as only a small amount of condensed liquid is circulated through the dilution refrigerator, the pressure that needs to be applied at the outflow of the cryogen pump is only about 350 millibar (mbar).

As an example of the cooling powers applied in this mode and in the high cooling power operation mode, the cooling power provided during the normal operation mode is about 1 microwatt (pW) at an operational temperature of about 10 mK; and is about 1 milliwatt (mW) at an operational temperature of about 100 mK during the high cooling power operation mode.

In the high cooling power operation mode, the pressure that needs to be applied at the cryogen pump outflow is about 1 bar due to the resistance to the flow imposed by the flow impedances. The higher pressure in the high cooling power operation mode than in the normal operation mode is primarily due to the change in flow rate from about 0.25 L/min in the normal operation mode to about 2.5 L/min in the high cooling power operation mode.

As a comparison, the pressure at the inflow of the cryogen pump is between about 100 mbar and 1 bar in each of these modes, and is about 0.01 mbar at the outflow of the dilution refrigerator.

To avoid loss of cryogen, the entire system through which the cryogen is passed is leak proof. As such, the high pressure remains when the dilution refrigerator is returned to room temperature because the cryogen pump (along with the other components) provides a gas tight seal to the external environment, so cryogen cannot otherwise escape from the system. Additional to this, the cryogen pump used in the embodiments of the cryogen pumping system described herein also provides an internal gas tight seal between the outflow of the cryogen pump and the inflow of the cryogen pump.

As the pump compresses the cryogen to generate the pressure required to pass it through the outflow pipe, the pressure in the inflow pipe is reduced. For example, as mentioned above, the pressure at the inflow will be in the region of about 100 mbar to about 1 bar, whereas the pressure at the outflow will be about 2 bar to about 3 bar at various stages during the operation. Thus, there is a pressure differential between the pressure of the cryogen in the inflow pipe and the pressure of the cryogen in the outflow pipe that is maintained while the refrigerator is inactive.

Accordingly, when the dilution refrigerator is next to be used, there will be a pressure differential across the pump. In order to alleviate this problem, in the embodiments described, the cryogen pumping system has a conduit to provide a means of equalising pressure across the pump.

Figure 1 shows a first embodiment of the cryogen pumping system 1. This shows a dual chamber double diaphragm pump 10 suitable for pumping cryogen. The pump has an inflow element 12 connected to one of the two chambers and an outflow element 14 connected to the other chamber. In an alternative embodiment, the diaphragm pump only has one chamber to which the inflow element and the outflow element are each connected.

As the pump 10 is in a room temperature environment, the pump and the cryogen passing through the pump is at a temperature of about 300 K. In use, the pump generates some heat by compressing the cryogen gas, but this does not raise the temperature of the pump of the cryogen gas significantly above the ambient temperature (i.e. by more than about 5 degrees Kelvin).

In the first embodiment, each of the inflow element 12 and the outflow element 14 is fluidly connected to the respective chambers of the pump 10. Each chamber is also fluidly connected to the other chamber by an intermediary pipe (not shown).

The inflow element 12 is also attached to an inlet pipe 22 and the outflow element 14 is attached to an outlet pipe 24. When in use, the inlet pipe supplies a cryogen (in gas form in a 3He/4He mix) to the pump 10 from a still (not shown) of a dilution refrigerator (not shown) and the outlet pipe transports the cryogen to a condenser (not shown) of the dilution refrigerator.

An example diameter of each of the inlet pipe and outlet pipe is about 10 mm. To regulate the pressure differential between the inlet pipe 22 and the outlet pipe 24, a conduit 30 is connected between the inlet pipe and the outlet pipe. The conduit has a bore 32 (shown in Figure 3 and Figure 5) through which cryogen is able to pass, allowing cryogen to pass between the inlet pipe 22 and the outlet pipe 24. This allows any pressure differential between the cryogen in the outlet pipe 24 and the cryogen in the inlet pipe 22 to equalise, removing the factor that causes the pump to stall when it is started.

Because high pressure is needed in the outlet pipe to push the cryogen through the pipe, the conduit also has a flow restriction 34 located in the middle of the conduit 30.

Embodiments of the flow restriction 34 are shown in Figure 3 and Figure 5, and are described in more detail below. Generally speaking however, the flow restriction provides a barrier 40 across the bore 32 through which the cryogen cannot pass apart from through a passage 42 through the barrier 40 that has a smaller cross-sectional area that the bore. For example, the diameter of the bore through the conduit 30 is about 10 mm, and the diameter of the passage is about 0.25 mm.

This means that to pass through the conduit 30, the cryogen must pass through the passage 42. As the passage has a smaller cross-sectional area than the bore 32, due to the Venturi effect, the cryogen pressure in the passage is lower than the cryogen pressure in the bore during flow of the cryogen through the conduit 30. Conversely, the velocity at which the cryogen passes through the passage is higher than the velocity at which the cryogen passes through the bore due to the same effect.

The drop in pressure and increase in velocity in the passage 42 causes the flow to approach a “choke” condition where the mass flow rate of cryogen through the passage is limited for non-compressible fluids and any further increase in the mass flow rate for compressible fluids is attenuated causing the mass flow rate through the passage to increase more slowly than the mass flow rate outside of the passage.

This restricts the ability of the conduit 30 to equalise the pressure between the inlet pipe and the outlet pipe when the pump is active and pumping the cryogen through the outlet pipe 24 at a higher mass flow rate than the mass flow rate that causes a choke condition in the passage 42 of the flow restrictor 34. This is because the mass flow rate of the cryogen in the outlet pipe, which at least in part causes the pressure differential between the cryogen in the inlet pipe and the outlet pipe, also causes the mass flow rate through the passage 24 through the barrier 40 of the flow restrictor to at least approach a choke condition. This limits the amount of cryogen that can pass through the flow restrictor stopping the pressure of the cryogen in the inlet pipe from equalising with the pressure of the cryogen in the outlet pipe.

We have found that by implementing this mechanism, the stall of the pump is prevented and there is an insignificant effect on the system performance, as the flow rates generated by the pump and the corresponding pressure remain almost unchanged. This can be seen from the plots shown in Figure 2.

Figure 2 shows a graph of pressure in millibar at the outflow of a cryogen pump against flow rate in standard cubic centimetres per minute (SCCM). The plot shown with square data points shows the results of measurements taken on a cryogen pumping system identical to that of the first embodiment, but without the conduit to equalise pressure between the inlet pipe and outlet pipe, and the plot with circular data points shows results of measurements taken on a cryogen pumping system according to the first embodiment as in Figure 1. The flow rate of each of these increase as the pressure increases.

It can be seen from Figure 2 that the flow rate is consistently lower for the cryogen pumping system of the first embodiment compared to the non-pressure equalised cryogen pumping system, and the difference increases (as can be seen from the fit applied to each plot) as the pressure increases. However, the difference in performance is only about 8.6%, as at 3 bar, the flow rates are 81 standard litres per minute (SLM) for the non-pressure equalised cryogen pumping system, compared to 74 SLM for a cryogen pumping system according to the first embodiment.

Figure 3 shows the first embodiment of the cryogen pumping system 1. This figure shows the pump 10 with the inflow element 12 and the outflow element 14. The inlet pipe 22 is connected to the inflow element 12. In use, cryogen passes along the inlet pipe in the direction indicated by arrow A towards the inflow element 12 and into the pump 10 from a still (not shown) of the dilution refrigerator.

The outlet pipe 24 is connected to the outflow element 14. In use, cryogen passes along the outlet pipe in the direction indicated by arrow B towards a condenser (not shown) out of the pump 10 from the outflow element 14. This motion of the cryogen is driven by the pump.

Each of the inlet pipe 22 and the outlet pipe 24 are connected to opposing ends of the conduit 30. This provides fluid communication between the bore 222 of the inlet pipe, the bore 32 of the conduit, and the bore 242.

As described above, the conduit 30 has flow restrictor 34 comprising the barrier 40. In this embodiment, the barrier 40 is a portion of the length of the bore 32 of the conduit that has a wall 402 with a greater thickness than the wall of the other portions of the conduit. Practically speaking, this is provided by an annular insert between two sections of pipe, which all together form the conduit 30 as is shown in Figure 1. The passage 42 is therefore formed from the bore of the annular insert.

When there is a pressure differential between the cryogen in the outflow pipe 24 and the cryogen in the inflow pipe 22, cryogen passes from the outlet pipe, into the conduit, through the passage 42 in the barrier 40, and into the inlet pipe 22 as indicated by arrow C.

When the pump 10 is inactive, the cryogen flow from the outlet pipe 24 to the inlet pipe 22 is able to equalise the pressure differential. When the cryogen is not being pumped round the system 1, this is achieved by only a moderate mass flow rate (for example, a flow rate of about 0.2 L/min at standard temperature and pressure) through the passage 42 in the conduit 30. This is because the volume of cryogen in each of the inlet pipe and the outlet pipe is small (for example, about 0.1 litre (L) in each of the inlet pipe and outlet pipe), which also allows the pressure differential to be equalised in a timescale commensurate with the switching time of the pump when under automatic control of a gas-handling system (not shown) used to control the pump and all other gas related elements of the wider system. The cryogen pump is able to start in a few hundred milliseconds, but the switching time referred to here is the time taken for the gas-handling system to switch the pump from a first state (such as active) to a second state (such as inactive) and back to the first state, which occurs over a period of a number of minutes.

In the first embodiment shown in Figure 3, when the pump 10 is active, the cryogen mass flow rate through the passage 42 in the conduit 30 becomes choked. This is caused by the flow velocity (V) of the cryogen reaching the sonic velocity (Vc) of the cryogen, which for helium is about 1000 metres per second (m/s), as the flow rate of the cryogen in the outlet pipe 24 is increased by the pump. This prohibits the mass flow rate from increasing further when there is any further increase in the flow rate in the outlet pipe 24 or increase in the pressure differential without a further increase in the mass flow rate being attenuated. This limits how much cryogen flows from the outlet pipe through to the inlet pipe 22. Accordingly, reduction in the increasing pressure differential between the inlet pipe and the outlet pipe is prevented when the pump is active.

Figure 4 shows the helium pressure ratio (i.e. the pressure differential between the cryogen pressure in the outlet pipe, indicated as P1, and the cryogen pressure in the inlet pipe, indicated as P2 and is representative of the cryogen used in the first embodiment) required across the flow restrictor 34 shown in Figure 3 to choke the flow through the flow restrictor. As can be seen from the plot, the required pressure ratio P2:P1 is about 1:2 giving P2/P1 as about 0.5.

Figure 4 shows that for a non-compressible fluid (such as a liquid cryogen), as the difference in pressure between the outlet pipe and the inlet pipe increases due to the increase in pressure in the outlet pipe, the flow velocity (V) of the cryogen through the flow restrictor 34 approaches the sonic velocity (Vc) of the cryogen at a smoothly decreasing rate as the pressure ration approaches about 1:2 of P2:P1. As the pressure in the outlet pipe increases further relative to the pressure in the inlet pipe, there is no further increase in the flow velocity of the cryogen through the flow restrictor, as the flow velocity stays equal to the sonic velocity.

For compressible fluids, such as a gaseous cryogen, the flow velocity also only increases to the sonic velocity as the pressure ratio increases, but the mass flow will continue to increase, as the pressure, and therefore the density, of the gaseous cryogen increase. However, the rate of increase of the mass flow rate will be smaller once the flow velocity reaches the sonic velocity in comparison with the rate of increase of the mass flow velocity before the flow velocity reaches the sonic velocity.

Figure 5 shows a second embodiment of the cryogen pumping system 1000. All of the features and functions of the second embodiment are identical to the features and functions of the first embodiment, apart from the flow restrictor 3400. As such, those features that are identical to those of the first embodiment are given the same reference numerals.

The flow restrictor 3400 of the second embodiment shown in Figure 5 comprises a plate 3402 as the barrier instead of a portion of the conduit with a thicker wall. The plate has an orifice 3404 through which cryogen is able to pass. Similar to the passage 42 of the first embodiment, an example diameter for the orifice is about 0.25 mm.

In this embodiment, the flow restrictor 3400 is a machined piece of solid stainless steel. This is a stainless steel cylinder that has been drilled out to form a thin layer of stainless steel as the plate and a hole drilled through the plate to form the orifice. This is then able to be connected to the rest of the conduit 30 in a similar manner to the annular insert of the first embodiment (such as with swaged joints).

Due to the form of the flow restrictor, in this embodiment, the plate is inflexible. In a variation of this embodiment, the plate is a flexible diaphragm with an orifice.

Due to the decreased thickness of the plate compared with the length of the portion of the conduit with a thicker wall in the first embodiment, flow through the orifice 3404 is never able to be full choked. This means that regardless of whether or not the cryogen is compressible or non-compressible, the flow rate will continue to increase once it reaches the sonic velocity, albeit at a lower rate than before it reaches the sonic velocity. As such, the plate 3402 with the orifice 3404 of the second embodiment has the same general effect as the flow restrictor 34 of the first embodiment in that it permits pressure equalisation when the pump is inactive, but prevents pressure equalisation when the pump is active.

In each embodiment, there is a range of physical structures that can be used for the components. For example, the pipes and the conduit may each be rigid or flexible, and may each be a single component or made up of multiple components secured together. The exact structure of these features will be decided by the user and will be influenced by the specific circumstances of the environment in which they are to be used. Stainless steel is an example of a suitable material for the components, such as the pipes and conduit.

As described above, the cryogen pumping system according to any embodiment is able to be fitted to a dilution refrigerator (not shown) to allow cryogen to be pumped around the dilution refrigerator as is described above. The dilution refrigerator is a standard dilution refrigerator.

The dilution refrigerator with the cryogen pumping system attached is able to be mounted within a cryostat (not shown). The cryostat is a standard cryostat, and the dilution refrigerator will usually be mounted in a neck of the cryostat with at least part of the dilution refrigerator being partially encircled by one or more radiation shields of the cryostat. When the dilution refrigerator is mounted within the cryostat, the pump of the cryogen pumping system will either be free standing or integrated into a rack of pumps along with other pumps required to operate the cryostat and dilution refrigerator.

Claims

1. A cryogen pumping system, comprising: a cryogen pump with an inflow element and an outflow element; and a conduit comprising a flow restriction, the conduit being connected between the inflow element and the outflow element, wherein the flow restriction is configured to limit the rate of cryogenic fluid flow through the conduit to less than the rate of cryogenic fluid flow producible by the cryogen pump when in use so as to maintain an operational pressure differential in the cryogenic fluid between the inflow element and outflow element produced by the cryogen pump when in use.

2. The system according to claim 1, wherein the cryogen pump prohibits internal fluid flow between the inflow element and outflow element when inactive.

3. The system according to claim 1 or claim 2, wherein the flow restrictor is a throttle.

4. The system according to any one of claims 1 to claim 3, wherein the flow restrictor is a static flow impedance.

5. The system according to claim 4, wherein the flow restrictor is a section of the length of the conduit with a narrower bore diameter than the other sections of the length of the conduit.

6. The system according to claim 4, wherein the flow restrictor is a sheet positioned across a bore of the conduit with an orifice therethrough, the orifice having a smaller cross-sectional area than the cross-sectional area of the bore of the conduit.

7. The system according to any one of the preceding claims, wherein the conduit is connected to the inflow element by an inflow pipe configured to supply cryogen to the pump when in use and is connected to the outflow element by an outflow pipe configured to transport cryogen away from the pump when in use.

8. A cryogen pumping system substantially as described herein, with reference to and as illustrated in the accompanying drawings Figure 1, Figure 3 and Figure 5.

9. A dilution refrigeration system, comprising: a dilution refrigerator; and a cryogen pumping system according to any one of the preceding claims comprising an inflow element and an outflow element, wherein the inflow element is connected to a cryogen outflow of the dilution refrigerator and the outflow element is connected to a cryogen inflow of the dilution refrigerator.

10. A dilution refrigeration system substantially as described herein.

11. A cryostat system, comprising: a cryostat; and a dilution refrigeration system according to claim 9 or claim 10 located within the cryostat and configured to provide cooling to at least a part of the cryostat.

12. A cryostat system substantially as described herein.